Project 3 seeks to address the high relapse rate of non-Hodgkin follicular lymphoma (FL) using immunocytokine (ICK) therapy combined with rituximab and single fraction radiation. We have developed and produced a clinical-trial grade immunocytokine composed of de-immunized anti-CD20 mouse monoclonal antibody (Leu16) fused with two IL2 molecules. DI-Leu16-IL2 (IND100885) has been administered to two patients with B-cell NHL and exhibits low toxicity with good immune activation. This project proposes three specific aims to assess the effectiveness and safety of DI-Leu16-IL2 therapy. Specific Aim 1: Perform a Pilot-Phase I study of DI-Leu16-IL2 combined with rituximab and single fraction involved-field radiation for treatment of follicular NHL. The primary objective of the trial is to select the schedule and dose of DI-Leu16-IL2. when given in combination with rituximab and single fraction radiation (XRT) that yields the more favorable immunologic response while maintaining safety. The secondary objective is to describe the safety, tolerability. and toxicity profile of DI-Leu16-IL2, given in combination with rituximab and XRT. Specific Aim 2: Perform a Phase II study of DI-Leu16-IL2 combined with rituximab and single fraction involved-field radiation for treatment of follicular NHL. In this aim. we will conduct a two-center Phase II trial of patients with FL who have relapsed or progressed after at least one prior regimen. The primary objective is to determine the anti-tumor activity of DI-Leu16-IL2 when combined with rituximab and single fraction radiation as assessed by overall response rate (CR+PR). Specific Aim 3: Evaluate the Biodistribution of DI-Leu16-IL2 using ^^^In labeled DI-Leu16-IL2. The primary goal of this study is to assess the biodistribution of DI-Leu16-IL2 using '^''in Dl-Leu16-IL2. The secondary goal is to obtain preliminary data that explores the possible association between DI-Leu16-IL2 uptake and FDG images (pre- and post- therapy) and tumor response. Significance: This non-transplant ICK-based therapy, if able to improve response duration for patients with follicular lymphoma, would have a significant clinical impact due to the increasing prevalence of this disease. The low toxicity profile of the combined treatment would be especially beneficial in older patients. Innovation: Our approach to targeted eradication of follicular lymphoma via the Dl-Leu16-1L2 immunocytokine in concert with single fraction local irradiation is novel in both its design and implementation. We have manufactured our own DI-Leu16-IL2, obtained FDA approval and commenced preliminary testing of this promising reagent. The addition of low-dose irradiation to this immunotherapy is hypothesized to enhance the immunologic response to DI-Leu16-IL2 treatment without significant additional toxicity.